With switching devices, in particular low-voltage switching devices, the current paths between an electrical supply device and loads and therefore their operating currents can be switched. Thus, by current paths being opened and closed by the switching device, the connected loads can be switched on and off safely.
An electrical low-voltage switching device, such as a contactor, a circuit breaker or a compact starter, for example, has one or more so-called main contacts, which can be controlled by one or else more control magnets, for the purpose of switching the current paths. In principle, in this case the main contacts include a movable contact link and fixed contact pieces, to which the load and the supply device are connected. In order to close and open the main contacts, a corresponding switch-on or switch-off signal is provided to the control magnets, whereupon the control magnets act with their armature on the movable contact links in such a way that the contact links complete a relative movement in relation to the fixed contact pieces and either close or open the current paths to be switched.
In order to provide better contact between the contact pieces and the contact links, correspondingly designed contact faces are provided at points at which the two meet one another. These contact faces include materials such as silver alloys, for example, which are applied at these points both on the contact link and the contact pieces and have a certain thickness.
The materials of the contact faces are subject to wear in each of the switching operations. Factors which can influence this wear are: the contact erosion or contact abrasion which increases with the increasing number of switching-on and switching-off operations, increasing deformations, increasing contact corrosion owing to the effect of arcs or environmental influences, such as vapors or suspended matter, for example, etc. As a result, the operating currents are no longer switched safely, which may lead to current interruptions, heating of the contacts or contact welding.
Thus, the thickness of the materials applied to the contact faces will be reduced in particular with the increase in contact erosion. The switching path between the contact faces of the contact link and the contact pieces therefore becomes longer, which ultimately reduces the contact force during closing. As a result, with an increasing number of switching operations the contacts no longer close correctly. Owing to the resultant current interruptions or else owing to increased switch-on bouncing of the contacts, the contacts may then be heated and therefore the contact material may be fused to an increasing extent, which in turn may lead to welding of the contact faces of the main contacts.
If a main contact of the switching device has been worn or even has welded, the switching device can no longer safely switch off the load. Thus, precisely in the case of a welded contact, at least the current path with the welded main contact will continue to carry a current or voltage, despite the switch-off signal, and the load is therefore not completely isolated from the supply device. Since, therefore, the load remains in an unsafe state, the switching device represents a potential source of faults.
As a result, for example in the case of compact starters according to IEC 60 947-6-2, in which an additional protective mechanism acts on the same main contacts as the control magnet during operational switching, the protective function may be blocked.
For the safe operation of switching devices and therefore in order to protect the load and the electrical system, such sources of faults therefore need to be avoided.